The prior art manual inspection methodology for inspection of containers such as vials, containing pharmaceutical injectables, for particle contamination, involves a two step particle inspection sequence. The first step is the inspection of the container for black or dark particles in front of a flat white background which is used to optimize the contrast of black particles. The second inspection step occurs in front of a flat black background to improve the contrast and thus the detection for white or light colored particles. In both cases, the container is manipulated to induce motion in any suspended particles to permit the human eye to distinguish between stationary container defects and moving contaminating particles.
In prior art practice, the container is moved about 8 to 12 inches between the white and black background inspection locations. Repositioning the inspected container requires both refocusing of the eyes and accurate repositioning of the inspector's head and the inspected container for each part of the inspection since the container is inspected at a near focus position, approximately 10 inches from the eye. These wasted motions reduce the rate at which an inspection of acceptable quality can be achieved. The energy lost in these non-productive motions result in deteriorated inspection performance due to the cumulative effect of inspector fatigue and can reduce inspector efficiency by as much as 30% at the end of a full work day, especially when a magnifying lens is used in the inspection. Repositioning can also vary the illumination available for inspection of the container, thereby introducing additional variability in the results of the inspection.
Accordingly, in said prior patent application, the disclosure of which is incorporated herein by reference thereto, the black and white backgrounds, in an optional preferred embodiment, are integrated in a quick change structure to eliminate such lost energy and variability.
A light inspection booth used in the prior art manual inspection procedures for contaminating particles in pharmaceutical products, comprises a 60 Hz ballast which excites a pair of 20 watt 11/2 inch diameter daylight fluorescent lamps arranged in an open lighting fixture above the inspection site. The prior patent application addresses improving the effect of lighting as used in the prior art, for enhancing inspection reliability.
The inspection for contaminating particles in injectable solutions has been shown to be probabilistic in nature, and with inspection performed with a fluorescent light source, a rough relationship between the probability of detecting a particle in a container and the particle size has been established.
In the standardized inspection conditions employed (without a magnifying lens), a 50 .mu.m particle was not detected, a 100 .mu.m particle was detected 70% of the time and a 200 .mu.m particle was consistently detected 100% of the time. As the light intensity employed for the inspection is increased or the contrast of the particle increases, the detection probability is increased, with the corollary being that reduced light intensity and contrast results in decreased detection probability.
The target of the manual inspection therefore is the visible particle size range greater than 100 .mu.m. The experimental rejection probability for all particles in this size range evaluates the effectiveness of the manual inspection. Extensive biophysics literature on human vision has established that the light intensity used for the inspection and the contrast of the target against the background determine both the rate and the accuracy with which a critical inspection can be successfully accomplished. Conversely, the wider the latitude of the illuminance employed for the inspection, the more variable will be the results of a manual inspection. With uncontrolled luminance variation the position of the inspected container with respect to the light source can multiply the difficulty of obtaining a secure inspection for contaminating particles.
With prior art light sources, the light intensity at the inspection point varies with the size of the inspected container and its position with respect to the light source. These factors modify the illuminance available for inspection of contaminating particles and thus the security with which these particles are detected.
To reduce the variability of human inspection results for contaminating particles in injectable fluids (or for that matter any type of similar illuminated inspection such as inspection for checking weld integrity and the like), the conditions under which the inspection is conducted must be defined and accurately controlled or contained.
The aforementioned co-pending application discloses a method and device for the accurate (with minimal variability) manual inspection of extended two dimensional surfaces or three-dimensional (not flat) samples, such as pharmaceutically injectable vials, for particle contamination, which samples are illuminated with diffused (non-point) light. Opposing vertically positioned and spaced illumination sources are provided above and below the sample, such as a vial, with the inspection volume (volume of the object, and movement space as may be required for inspection) being positioned at and around the illumination midpoint (lumen light balance) between the illumination sources for minimal illumination variability as a function of distance from the illumination midpoint. The position of the illumination mid-point for the inspection volume is adjustable according to the eye level of the particular inspector, whereby the illumination midpoint is brought into alignment with such eye level. Since the illumination midpoint, for light sources of equal intensity, is also the physical midpoint therebetween, adjustment is readily physically effected. Thus, lighting at any point and angle relative to the inspection volume (and in a plane parallel to the inspector) is possible, as long as the lighting is symmetrically balanced to provide the requisite illumination mid-point.
In order to further enhance inspection security and to reduce inspector fatigue there is no movement away from the designated inspection volume during any part of the inspection. Accordingly, the inspection is effected in front of a background which is automatically and quickly variable between light and dark to permit for optimal contrast for inspection for light and dark particles while the vial is at the inspection point within the inspection volume.
In prior art procedures, during the inspection against a white background, the contrast available for the detection of white particles is diminished. Similarly during the inspection against a black background, the contrast available for the detection of black particles is diminished. Detection benefits made possible by such procedures entail detection disabilities in 50% of the inspection for contaminating particles in injectable solutions.
By using the sequential test procedure described, inspectors can achieve a Reject Zone Efficiency (i.e., a reject rate of visible particle contaminated containers of about 85 to 90%). However, attempts to achieve inspection accuracy beyond this level result in an exponential increase in both real and false reject rates.